1. Field of the Invention
This invention generally relates to plastic screw extruders; and more specifically, to a screw extruder designed to adjust easily the relative rates at which a plastic material is pressurized, heated and mixed as that material passes through the extruder, and to a method of operating such an extruder.
2. Description of the Prior Art
Typically, a plastic screw extruder comprises an elongated cylinder or barrel, a feed hopper mounted on one end of the barrel, a rotatable screw axially extending in the barrel, and a plurality of heaters provided around the outside walls of the barrel. In use, a plastic material, such as a polymer in the form of solid pellets or chips, is fed into the hopper, which in turn feeds that material into the extruder barrel. The screw in the barrel is rotated to advance the plastic therethrough, and the heaters around the barrel are activated to heat the plastic as it advances therein. This heat and the frictional heat from the rotating screw cause the plastic to change from the solid state to a molten state, and the molten material is then forced from the barrel and through a die that forms the molten plastic into a desired shape. This formed material can be subjected to subsequent shaping operations and, for example, a plastic that is extruded in the shape of a tube can later be expanded in a blow molding operation to form a plastic bottle.
In this extrusion operation, the extruder screw mixes, pressurizes, and heats the plastic material advancing through the extruder. All of these parameters affect the output of the extruder, and it is thus desirable to obtain certain pressure, temperature and mixing levels in order to obtain a certain output from the extruder. The degrees to which the extruder screw mixes, pressurizes and heats the plastic material are all interdependent, however, so that changing the level of one of these parameters normally affects the levels of the other parameters. As a result, in practice, any particular chosen combination of such parameters is usually a compromise between optimum levels of pressure, temperature and mixing.
In a given extrusion process, a given extruder screw will normally produce a particular combination of pressure, temperature and mixing; but it is often very difficult, if not practically impossible, to predict in advance what that particular combination will be. Thus, selecting the proper screw for a particular extrusion process is normally the result of trial and error; that is, trying a number of different screw designs to see which one produces, or comes closest to producing, the desired combination of pressure, temperature and mixing. Each time a different screw is tried, though, the extruder must be taken apart, the screw replaced with one of a different design, and then the extruder reassembled. As will be understood by those skilled in the art, this is a time consuming and expensive procedure.
Moreover, often a particular extruder barrel will be used in a number of different extrusion processes. Usually, each time an extruder barrel is employed in a different process, the extruder screw must be replaced with another screw in order to obtain the preferred mixing, pressure and temperature levels for the new process. Even when it is known what screw will produce that new combination of conditions, replacing the extruder screw with one having a different design is a laborious and costly procedure.
Various prior art plastic screw extrusion methods and apparatus are disclosed in U.S. Pat. Nos. 4,197,070; 4,214,859; 4,289,410; and 4,300,840.
U.S. Pat. No. 4,197,070 discloses a control system for a plastic extruder which automatically controls both the melt pressure and the melt temperature of the plastic material at the outlet of the extruder barrel.
U.S. Pat. No. 4,214,859 describes an arrangement for adjusting the output quantity of a screw extruder by varying the cross-sectional area of the extruder cylinder available for plastic flow. This is done to match the output quantity of the extruder to the output quantity of a polymerization system that feeds the extruder.
U.S. Pat. No. 4,289,410 discloses a plastic screw extruder comprising a cylinder and a screw, each of which includes a plurality of vertically divided segments. Individual screw segments can be replaced by removing the adjacent cylinder segment, and it is not necessary to completely disassemble the cylinder or to completely remove the screw therefrom.
U.S. Pat. No. 4,300,840 discloses apparatus for extruding resinous material including a cylinder and a screw located therein for advancing the resinous material through the cylinder. A plurality of axially extending recesses are formed in the interior surface of the cylinder to assist forward movement of the resinous material therethrough.
These prior art references address various problems or aspects of screw extrusion processes. However, even with these prior art arrangements, selecting the proper screw design and changing the extruder screw are normally time consuming and expensive tasks.